The present invention relates to dampening means in apparatus for regulating fluid pressure and more particularly to deadweight pneumatic testers.
Pneumatic deadweight testers are utilized for creating a known, precise pressure in a pneumatic line. The deadweight tester is utilized to calibrate less expensive pressure measurement apparatus such as orifice meters. A significant application of orifice metering is measuring gas flow in pipelines. Both gas production and interstate delivery of gas may be measured. Accurate measurement is useful for both commercial purposes and for compliance with governmental production regulations of such hydrocarbon production regulating administrative entities such as the Texas Railroad Commission. Producers and pipeline operators can face regulatory sanctions for failure to maintain an adequate number of pressure measurement means and adequate periodic calibration. A very small percentage error in measurement of volume of gas delivered can mean a very large dollar error in its cost.
A common form of pressure meter is the bellows type manometer. This apparatus is commonly used to measure differential pressures corresponding to 20, 50, 100 and 200 inches of water. A body of literature covers the significance and requirements for proper measurement in the field. The American National Standards Institute has published ANSI-API 2530, Standard Orifice Metering of Natural Gas (formerly American Gas Association Report No. 3). Work in this area was begun by the American Gas Association in May 1924. Their report No. 1 was issued in 1930. Revisions preceding the current Standard were made in 1935, 1955 and 1977.
Not only does the Standard specify a procedure for calibration of orifice meters, it provides for a procedure for witnessing of calibration. The gas business commonly requires a witness to insure the accuracy and validity of measurement and physical tests. Documentation by the individual calibrator is insufficient. One of the minimum essential items necessary for a witness to perform his function properly is a deadweight tester.
A well-known form of deadweight tester widely used in the art is disclosed in U.S. Pat. No. 3,047,005 issued July 31, 1962 to R. J. Karr for Pressure Regulator. In that system, a vertically disposed nozzle is provided in which a ball rests on an arcuate seat. Input air enters through the nozzle and exits in annular passage surrounding the ball to lift the ball. Air exits through a vertical passage coaxial with the axis of the nozzle. A spider comprising a weight support rests on the ball. Karr explains that effective area of the ball is known and is substantially constant. The weight of the ball and the weight carrier are also constant. It is only necessary to place weights of a correct value on the weight carrier which combined with the weight of the ball and the carrier to produce the desired pressure in the outlet pneumatic line.
However, regulating apparatuses may have a resonant frequency. As Karr points out, the apparatus described may have a self-generating, self-oscillatory action that may tend to produce hunting of the outlet pressure in the exit line downstream of the ball. This would happen due to operation of the pressure regulating means which operate to keep the vertical position of the ball constant for a given total weight carried thereon. A feedback line in the pressure regulator would cause the regulator to reduce input pressure when the ball rises. However, the opposite should occur because by the time pressure is decreased, the ball will be falling. At such a time, an increase in pressure is desired in order to absorb the momentum of the ball and dampen its oscillation. Karr solves this problem by utilizing what is now well-known dampening means. A tank and coil arrangement are provided in which the outlet line enters a tank, tubing is wrapped around the access of the tank in a helical disposition for a given length and terminates within the tank. The outlet line continues at an outlet from the tank. The tank and coil arrangement gives the desired out-of-phase reaction to damp oscillations in the positioning of the ball.
As Karr points out, the optimum coil length may be determined empirically for each application. In a nominal application in which one-eighth inch tubing enters a nozzle six inches long, the dampening tank may be six inches in diameter and two inches deep. The coil therein may be a grand total of twelve feet long in sections of stepped diameter, for example 1/32 inch, 1/16 inch, 3/32 inch, and 1/8 inch. This construction is complex in that a great deal of tubing bending must be performed, different diameters of tubing must be assembled together and great care must be exercised not to produce kinks in the tubing in the bending operation. The size of the deadweight tester is also increased. This is disadvantageous in the field. Further, should liquid enter the system, they are not easily cleared from a long, small diameter length of tubing. Further, such dampening means are not readily adjustable to respond to new conditions. It is therefore desirable to provide a system with improved dampening means.